The migration of luteinizing hormone-releasing hormone neurons in the developing rat is associated with a transient, caudal projection of the vomeronasal nerve.

Luteinizing hormone-releasing hormone (LHRH) neurons originate in the olfactory placode and vomeronasal organ and migrate to the brain from embryonic day 14 (E14) in the rat. We investigated the development of the vomeronasal nerve and its role as a guide for migrating LHRH neurons. Using fluorescent, lipophilic dye tracing methods, we observed axons that emerge from the vomeronasal organ and cross the nasal septum as several large fascicles. At E14-15, these fascicles converge as they enter the region of the cribriform plate and subsequently disperse, projecting dorsally and caudally across the olfactory bulb and rostral forebrain. At E16, the dorsal branch of the vomeronasal nerve forms a more tightly fasciculated projection; the caudal fibers remain dispersed, extending along the medial forebrain. The number of caudally directed axons decreases during development, leaving four or five present at postnatal day 4 (P4). Immunohistochemical studies indicate that the vomeronasal nerve can be divided into four spatially distinct subpopulations of fibers. One subset, composed of caudal fibers that terminate in the lamina terminalis, selectively expresses TAG-1, a transient axonal surface glycoprotein and PSA-N-CAM, a highly polysialated form of neural cell adhesion molecule. The extension and subsequent retraction of this branch of the vomeronasal nerve corresponds spatially and temporally with the migration of LHRH neurons from the nasal cavity to the brain. Our studies show that between E14 and E18, LHRH neurons migrate in contact with the TAG-1+, PSA-N-CAM+ caudal branch of the vomeronasal nerve.